/**
 * Friendly Interactive Recursion Educator (FIRE)
 * COMS 4115: Programming Languages & Translators
 * Prof. Aho
 * 
 * @author Brian Lu
 * @author Sahar Hasan
 * @author Steve Hehl
 * @author Tristan Naumann
 * @version 2009-05-12
 */
package firesymbols;

/**
 * NullTypeSymbol is a singleton class representing FIRE's
 * null type which is used for type checking.
 */
public class NullTypeSymbol extends Symbol {
	
	private static NullTypeSymbol instance = null;
	
	private NullTypeSymbol() {
		super("null");
	      // Exists only to defeat instantiation.
	}
	
	public static NullTypeSymbol instance() {
	
		if(instance == null) {
	         instance = new NullTypeSymbol();
		}
		return instance;
	}
}
